In recent years, group III-nitride based deep UV light-emitting diodes (LEDs) operating at wavelengths from 250 to 350 nm have attracted growing interest, due to their promise of applicability in various fields including biological agent detection, disinfection, covert communications, optical data storage, and solid state lighting. Several groups including ours have recently reported results for sub-290 nm LEDs grown over sapphire substrates as described in V. Adivarahan, S. Wu, J. P. Zhang, A. Chitnis, M. Shatalov, V. Mandavilli, R. Gaska, and M. Asif Khan, Appl. Phys. Lett. 84, 4762 2004; A. J. Fischer, A. A. Allerman, M. H. Crawford, K. H. A. Bogart, S. R. Lee, R. J. Kaplar, W. W. Chow, S. R. Kurtz, K. W. Fullmer, and J. J. Figiel, Appl. Phys. Lett. 84, 3394 2004; A. Hanlon, P. M. Pattison, J. F. Kaeding, R. Sharma, P. Fini, and S. Nakamura, Jpn. J. Appl. Phys., Part 2 42, L628 2003; K. H. Kim, Z. Y. Fan, M. Khizar, M. L. Nakarmi, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 85, 4777 2004 and A. A. Allerman, M. H. Crawford, A. J. Fischer, K. H. A. Bogart, S. R. Lee, D. M. Follstaedt, P. P. Provencio, and D. D. Koleske, J. Cryst. Growth 272, 227 2004.
Recently, device structure and growth optimization efforts have resulted in 280 nm LEDs providing 1 mW of output power at 20 mA of drive current with an external quantum efficiency of 1% as described in W. H. Sun, V. Adivarahan, M. Shatalov, Y. B. Lee, S. Wu, J. W. Yang, and M. Asif Khan, Jpn. J. Appl. Phys., Part 2 43 L1419 2004.
In spite of these impressive power numbers, there still is considerable work required to realize high efficiency devices. This not only involves material development, structural improvements, and process optimization but also the realization of robust packaging schemes to provide effective thermal management to allow more efficient device operation and to maintain long-term device performance. There have been various reports on problems with device self-heating effects in packaged III-N deep UV LEDs which lead to the early saturation of optical power as the current was increased, basically corresponding to an increase in device temperature as described in A. Chitnis, S. Jason, V. Mandavilli, R. Pachipulusu, S. Wu, M. Gaevski, V. Adivarahan, J. P. Zhang, M. Asif Khan, A. Sarua, and M. Kuball, Appl. Phys. Lett. 81, 3491 2002; A. Sarua, M. Kuball, M. J. Uren, A. Chitnis, J. P. Zhang, V. Adivarahan, M. Shatalov, and M. Asif Khan, Mater. Res. Soc. Symp. Proc. 743, L7.8.1 2002 and A. Chitnis, V. Adivarahan, J. P. Zhang, S. Wu, J. Sun, R. Pachipulusu, V. Mandavilli, M. Gaevski, M. Shatalov, and M. Asif Khan, Electron. Lett. 25, 1709 2002.
Removal of by-product heat from a III-nitride light emitting diode and laser diode during operation is important since the lifetime of such a device is a strong function of operating temperature, with an increased operating temperature resulting in a reduced lifetime for the device. For example, the lifetime of a conventional gallium nitride based light emitting diode (LED) and laser diode (LD) will decrease by more than an order of magnitude by rise in temperature. Accordingly, simple heat transfer considerations suggest that a light emitting diode should be in contact with a heat sink material of the highest possible thermal conductivity in order to remove by-product heat as quickly as possible. However, if there is a mismatch between the coefficient of thermal expansion of the light emitting diode and that of the heat sink, the brittle semiconductor diode will experience mechanical stress during operation as a consequence of by-product heat production, and the magnitude of this stress will be a function of the size of the mismatch. Such stress can cause device failure within a few hours if it is severe and uncompensated.
This problem becomes more severe for devices operating deeper in the UV region, especially less than 340 nm, where the external quantum efficiency is typically 1-4% or less. Hence, an effective thermal management system is necessary to ensure efficient heat removal from these LEDs in order to allow operation with sufficiently high bias levels. Earlier studies indicate that flipping the LED chip over and mounting it to the header p-side down greatly improves the transfer of heat away from the junction. Though this is beneficial, the gains with this technique are still inadequate for achieving high efficiency.
Throughout the art amorphous AlN is used as the primary carrier submount. The thermal conductivity of amorphous AlN is typically 70-90 W/mK. The amount of heat generated in the devices flow to submount which then starts to heat up due to conduction. Thus for high power, high current deep UV LEDs it is imperative to improve the heat conduction through these submounts/carriers.
An optimized thermal management and device package mechanism is particularly important for use in the fabrication of highly efficient semiconductor ultraviolet light emitting diodes. Such devices contain a p-n junction which forms a diode, and this junction functions as the active medium of the LED. The efficiency of such light emitting devices in converting electrical power to output optical radiation is relatively high and, for example, can be in excess of 50 percent. However, electrical power which is not converted to light by such a device is lost as heat.
However, for a semiconductor based especially III-N based deep UV LEDs have their efficiency lower in the range of 30-40%. This is primarily due to an increased number of defects for such epilayers. Moreover, due to ineffective doping, the electrical carriers in such epilayers are relatively low as compared with visible LED. Thus, the device possesses higher ON-resistance which then leads to increased device joule heating thus lowering the overall efficiency which is lost as heat. The already lower efficiency and increased generation of heat causes such light emitting diodes with poor overall efficiency in terms of lumens per watt.
It has been an ongoing desire to provide a UV or deep UV LED which is efficient with regards to the lumens per watt. The present invention provides an LED, and method for making the LED, which is highly efficient.